1. Field
The present disclosure relates to optical communication equipment and, more specifically but not exclusively, to the use of forward-error-correction techniques for carrier-phase estimation in optical transport systems.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Advanced modulation formats, such as quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM), enable data to be modulated in both the phase and the amplitude of the optical carrier wave. However, a known drawback of these modulation formats is a relatively complex receiver structure, where the relative phase between the optical local-oscillator (OLO) signal and the carrier wave of the received modulated optical signal needs to be tracked continuously for the modulated data to be recovered. The tracking can be done, e.g., using a phase-lock loop (PLL) circuit, which is a negative-feedback control system configured to lock the OLO signal to the phase and frequency of the carrier wave of the received modulated optical signal. However, PLL circuits may add significantly to the cost and complexity of the corresponding coherent optical receiver. An alternative to a PLL circuit is the use of intradyne detection, where the phase tracking is implemented in the electrical-digital domain via digital signal processing after optical-to-electrical conversion, sampling, and analog-to-digital conversion. However, efficient synchronization and phase-estimation algorithms for intradyne detection are still undergoing research and development.